With the rapid expansion of the market for laptop computers, mobile phones, electric vehicles, and the like, a secondary battery having a high energy density is expected. Examples of a method for obtaining a secondary battery having a high energy density include a method in which a negative electrode material having a large capacity is used, and a method in which an electrolyte liquid having an excellent stability is used.
As for the negative electrode material, using silicon or silicon oxide as a negative electrode active material has been tried as disclosed in, for example, Patent Documents 1 to 3. Patent Document 1 discloses using silicon oxide or a silicate as a negative electrode active material of a secondary battery. Patent Document 2 discloses a negative electrode for a secondary battery which has an active material layer containing a carbon material particle that can absorb and desorb lithium ion, a metal particle that can be alloyed with lithium, and an oxide particle that can absorb and desorb lithium ion. Patent Document 3 discloses a negative electrode material for a secondary battery which is formed by coating the surface of a particle, which has a structure in which a silicon fine crystal is dispersed in a silicon compound, with carbon. The negative electrode active materials described in Patent Documents 2 and 3 have an effect of reducing volume change as the entire negative electrode.
As for the electrolyte liquid, using an electrolyte liquid containing another compound in addition to a conventional non-aqueous electrolyte liquid has been tried as disclosed in, for example, Patent Documents 4 to 6.
Patent Document 4 discloses using an electrolyte containing an asymmetric non-cyclic sulfone and a chain-type ester compound.
Patent Documents 5 and 6 disclose using a non-aqueous electrolyte liquid containing a fluorinated ether.
Patent Document 7 discloses, in the Examples, an electrolyte liquid containing any compound among fluorine-containing ethers, fluorine-containing esters, and fluorine-containing carbonates.
Patent Documents 8 and 9 disclose an electrolyte liquid containing a fluorine-containing ester compound.
Also, examples of the method for improving energy density of the battery include not only using an active material that has high capacity, but also increasing an operating potential of the battery and improving charge/discharge efficiency, cycle life, or the like. From among these methods, a method that increases an operating potential of the battery is effective for downsizing and weight-saving a battery module used for electronic vehicles or the like because an assembled battery having a smaller number of serially-connected batteries than a conventional assembled battery can be provided.
As a 4 V class positive electrode active material for a lithium ion secondary battery, lithium cobaltate and lithium manganite (average operating potential: 3.6 to 3.8 V, with respect to lithium potential) are known. In contrast, as a 5 V class positive electrode active material, for example, compounds (average operating potential: 4.6 V or more, with respect to lithium potential) obtained by substituting Mn of a spinel type lithium manganate by Ni, Co, Fe, Cu, Cr, or the like are known.
For example, the capacity of LiNi0.5Mn1.5O4 that is a 5 V class active material is 130 mAh/g or more, and the average operating voltage is 4.6 V or higher with respect to metal Li, and the material is expected as a material having high energy density. Further, the spinel type lithium manganese oxide is advantageous in that it has a three-dimensional lithium spreading path, in that it has thermodynamic stability higher than the other compound, and also in that it can be easily synthesized.
For example, Patent Documents 10 or 11 disclose a secondary battery in which a fluorinated solvent is used in the case of using a positive electrode active material which has charging and discharging field of 4.5 V or higher.